1. Field of the Invention
The present invention relates to methods and apparatus for synchronization of a received signal, and in particular to methods and apparatus for use in telecommunications networks. The present invention also relates computer program products for synchronization to a received signal in which data is sent in frames and training sequences are provided for synchronization. The present invention particularly relates to telecommunications networks in which data is sent in frames and training sequences are provided for synchronization, e. g. especially to OFDM and COFDM telecommunications systems.
2. Technical Background
There are many forms of known telecommunications systems including wireless based and wireline systems. Such systems may be used to transfer voice or data systems across a variety of channels, e. g. satellite, optical fibre, coaxial cable, cellular wireless, point-to-point microwave systems. In general there is a transmitter for transmitting a signal and a receiver for receiving the signal as part of the system. To improve reception, the transmitted signal may be coded in a variety of ways. A digital signal received at a receiver must be synchronized in some way in order to extract any message conveyed in the signal. There are various ways in which synchronization can be achieved. For instance, a known symbol sequence (e. g. a training symbol sequence) may be correlated with a received signal known to contain the same sequence. This may be called cross-correlation. Training sequences are widely used for synchronization. Alternatively, if the transmitted signal includes a repeated or cyclic sequence, such as a cyclic symbol prefix as can occur in OFDM (Orthogonal Frequency Division Multiplex) systems, the cyclic sequence may be autocorrelated with the same prefix received at a different time.
Such synchronization methods are known, for instance, from “Robust Frequency and Timing Synchronization for OFDM”, Scmidl and Fox, IEEE Trans. On Communications, vol. 45, no. 12, Dec. 1997 and “On Synchronization in OFDM Systems using the cyclic prefix”, Jan-Jaap van de Beek, Magnus Sandfell, Per Ola Börjesson, Proc. of the RVK 96, pages 663-667, Lulea, Sweden, June 1996.
OFDM has been proposed for various wireless telecommunications systems such as the IEEE 802.11a standard, the ETSI High Performance Local Area Network Type 2 (HIPERLAN/2), the ETSI Digital Audio Broadcasting (DAB) standard, and the pan-industry Digital Video Broadcasting (DVB) project.
Synchronization can become more difficult when there is a clock offset between the transmitter clock and the receiver clock. The channel across or through which the data is transmitted may distort received signals which may make synchronization more difficult. In radio systems there may be multiple paths between the transmitter and receiver which result in receipt of multiple signals delayed with respect to each other depending upon the length of the path. In the presence of channels having long impulse response times (that is, those in which the impulse response time is comparable to the length of the training sequence or the cyclic sequence), the accuracy of synchronization drops. Intersymbol Interference(ISI) becomes worse when the impulse response time is long and this can have a negative effect upon synchronization and, as a result, on the operation of a receiver. A further problem, especially with OFDM systems is carrier frequency offset. OFDM systems are more sensitive to frequency offset and phase noise than single carrier systems. In an OFDM system the subcarriers are perfectly orthogonal only if the transmitter and receiver use exactly the same frequencies. Any frequency offset results in Intercarrier Interference. Hence, frequency offset must be minimized. A related problem is phase noise. A practical oscillator does not produce a carrier at exactly one frequency, but rather a carrier that is phase modulated by random phase jitter. As a result the received frequency is never constant. The received signal may also contain general noise, e. g. white Gaussian noise.
The first part of a typical OFDM frame comprises a preamble, for example aHIPERLAN/2 preamble consists of a short (STS) and a long training sequence (LTS). The 10 STS contains repetitions of a training symbol with duration of 800 ns on 12 subcarriers. Each of the symbols is a quarter of the duration of the part of a normal data symbol analyzed by the Fast Fourier Transform. Each data symbol of an OFDM signal has a cyclic prefix, i.e., the first To seconds part of each OFDM symbol is identical to the last part. The preamble also includes a long training sequence which two data symbol and a cyclic prefix. The STS may be used for coarse frequency estimation whereas the LTS may be used for precise frequency estimation. The STS may also be used for symbol timing estimation by cross-correlation.
An object of the present invention is to provide a method and apparatus for improved synchronization of a received signal.
A further object of the present invention is to provide a method and a system which allows robust synchronization even under extreme conditions.
Still a further object of the present invention is to provide a method and a system which allows synchronization with lower risk of perturbation caused by intersymbol interference.
Yet a further object of the present invention is to provide a method and a system which allows reception with a better bit or symbol error rate.
Still another object of the present invention is to provide a method and a system which allows reception with a higher transmission rate.